I was drawing her from behind in the second panel, and Martin had already described Misty as being on the chubby side. So as I drew her from behind, with her butt facing the camera, I figured it'd be neat to give her a large one.

So I asked Martin, word-for-word, "I have decided that Misty will have a gigantic ass while I am artist. Do I have approval?" He said yes, and there it is.

Incidently, while I don't mind a sizable booty, I only gave her a big one because I thought it'd be funny. I apparently have the maturity level of a seven-year-old.